1. Threads and Fasteners

2. Two Categories of Fasteners
Temporary and Permanent

3. Summary
How to represent fasteners and threads on an engineering drawing.
How to calculate bolt and screw clearance holes.
Threads are represented by thread symbols, not by a realistic drawing.

4. Fasteners Fasteners include:
bolts and nuts (threaded)
set screws (threaded)
washers
keys
pins
Fasteners are not a permanent means of assembly such as welding or adhesives.

5. Fasteners
Fasteners and threaded features must be specified on your engineering drawing.
Threaded features: Threads are specified in a thread note.
General Fasteners: Purchasing information must be given to allow the fastener to be ordered correctly.

6. Thread Definitions
Screw Thread: A ridge of uniform section in the form of a helix.

7. Thread Definitions
External Thread: External threads are on the outside of a member.
A chamfer on the end of the screw thread makes it easier to engage the nut.
Chamfer

8. Thread Definitions External Thread:
An external thread is cut using a die or a lathe.

9. Thread Definitions
Internal Thread: Internal threads are on the inside of a member.
An internal thread is cut using a tap.

10. Thread Definitions
Major DIA (D): The largest diameter (For both internal and external threads).
Minor DIA (d): The smallest diameter.
Depth of thread: (D-d)/2
Pitch DIA (dP): The diameter at which a line cuts the spaces and threads equally.

11. Identify the Major, Minor & Pitch diameters and the Thread Depth.3 2 1 4 5 6 7 8 8 10 9

12. Identify the Major, Minor & Pitch diameters and the Thread Depth.3 2 1 4
Thread Depth
Minorn Pn
Majorn 8 10 9

13. Thread Definitions Crest: The top surface. Root: The bottom Surface.
Side: The surface between the crest and root.

14. Identify the Crest, Root and Side.3 2 1 4
Thread Depth
Minorn Pn
Majorn 8 10 9

15. Identify the Crest, Root and Side.2 1
Root
Thread Depth
Minorn Pn
Majorn 8 10
Side

16. Thread Definitions
Pitch (P): The distance from a point on a screw thread to a corresponding point on the next thread (in/Threads).
Angle of Thread (A): The angle between the threads.

17. Thread Definitions Screw Axis: The longitudinal centerline.
Lead: The distance a screw thread advances axially in one turn.

18. Identify the Pitch, Screw Axis and Thread Angle.
Crest 2 1
Root
Thread Depth
Minorn Pn
Majorn 8 10
Side

19. Identify the Pitch, Screw Axis and Thread Angle.
Crest
Pitch
Axis
Root
Thread Depth
Minorn Pn
Majorn 8
Angle
Side

20. Thread Definitions
Right Handed Thread: Advances when turned CW. (Threads are assumed RH unless specified otherwise.)
Left Handed Thread: Advances when turned CCW.

21. Application Question Name an example of a left handed thread.
Left peddle of a bike
Threads on a propane tank connection

22. Types of Thread
There are many different types of thread forms (shape) available. The most common are;
Unified
Metric

23. Types of Thread Thread form choice depends on;
what it will be used for
length of engagement
load
etc…

24. Types of Thread (Form) Thread Name Figure Uses Unified screw thread
General use.
ISO metric screw thread
Square
Ideal thread for power transmission.

25. Types of Thread (Form) Thread Name Figure Uses ACME
Stronger than square thread.
Buttress
Designed to handle heavy forces in one direction. (Truck jack)

26. Manufacturing Threads
Internal Threads
First a tap drill hole is cut with a twist drill.
The tap drill hole is a little bigger than the minor diameter. Why?

27. Manufacturing Threads
Incomplete threads
Internal Threads
Then the threads are cut using a tap.
The tap drill hole is longer than the length of the threads. Why?

28. Drill Sizes

30. Manufacturing Threads
Internal Threads
Chamfers are sometimes cut to allow for easy engagement.

31. Manufacturing Threads
External Threads
You start with a shaft the same size as the major diameter.

32. Manufacturing Threads
External Threads
The threads are then cut using a die or on a lathe.

33. Drawing Screw Threads
There are three methods of representing screw threads on a drawing.
Detailed
Schematic
Simplified
Screw thread representation present in this chapter is in accordance with the ASME Y standard.

34. Detailed Representation
A detailed representation is a close approximation of the appearance of an actual screw thread.

35. Detailed Representation
Pros and Cons?
Pro: Looks good and clearly represents a thread.
Con: Takes a long time to draw.

36. Schematic Representation
The schematic representation uses staggered lines to represent the thread roots and crests.

37. Schematic Representation
Pros and Cons?
Pro: Nearly as effective as the detailed representation and easier to draw.
Con: Still takes some time to draw.

38. Schematic Representation
Rules of use for Schematic threads
Should not be used for hidden internal threads or sections of external threads.

39. Simplified Representation
The simplified representation uses visible and hidden lines to represent the major and minor diameters.

40. Simplified Representation
Pros and Cons?
Pro: Simple and fast to draw.
Con: Doesn’t look like a thread.

41. Simplified Internal Threads

42. Simplified Internal Threads

43. Drawing Screw Threads
Thread tables in the appendix can be used to look up value for the;
Pitch
Minor diameter
Tap drill diameter
If screw thread tables are not available, the minor diameter can be approximated as 75% of the major diameter.

44. Unified Threads (inch)
After drawing a thread, we need to identify the size and thread form in a thread note.
Thread Note

45. Unified Thread Note Components

46. Unified Threads (inch)
Major Diameter: The largest diameter.
Threads per inch: Number of threads per inch for a particular diameter.
Equal to one over the pitch (1/P).

47. Unified Threads (inch)
Thread Form and Series: The shape of the thread cut.
UNC = Unified National coarse.
For general use.
UNF = Unified National fine.
Used when high degree of tightness is required.
UNEF = Unified National extra fine.
Used when length of engagement is limited (Example: Sheet metal).

48. Unified Threads (inch)
Thread Class: Closeness of fit between the two mating threaded parts.
1 = Generous tolerance. For rapid assembly and disassembly.
2 = Normal production
3 = High accuracy

49. Unified Threads (inch)
External or Internal Threads
A = External threads
B = Internal threads
Right handed or left handed thread
RH = Right handed (right handed threads are assumed if not stated.)
LH = Left handed

50. Unified Threads (inch)
Depth of thread: The thread depth is given at the end of the thread note and indicates the thread depth for internal threads
This is not the tap drill depth.

51. Unified Threads (inch)
Thread class is assumed to be 2.
Threads are assumed to be RH.
May be left off if assumptions hold.

52. Exercise 5-2
Identify the different components of the following Unified National thread note.
1/4 – 20 UNC – 2A – RH
1/4 20
UNC 2 A RH
.25 inch Major DIA
20 threads per inch (P = 1/20 = .05)
Thread form & series – UN Coarse
Thread Class – Normal Production
External Threads
Right Handed Threads

53. Exercise 5-2
Identify the different components of the following Unified National thread note.
1/4 – 28 UNF – 3B – LH
1/4 28
UNF 3 B LH
.25 inch Major DIA
28 threads per inch (P = 1/28 = .036)
Thread form & series – UN Fine
Thread Class – High Accuracy
Internal Threads
Left Handed Threads

54. Unified National Thread Tables
Standard screw thread tables are available in order to look up the:
Major diameter
Threads per inch
Minor diameter or Tap drill size.
Thread tables are located in the Appendix.

55. Exercise 5-3
Write the thread note for a #10 fine thread. (See Appendix)

56. Exercise 5-3
Write the thread note for a #10 fine thread.
10 – 32 UNF

57. Exercise 5-3
Write the thread note for a #10 fine thread. (See Appendix)
Is the major diameter 10 inches? No
10 – 32 UNF

58. Exercise 5-3
Write the thread note for a #10 fine thread. (See Appendix)
Is the major diameter 10 inches?
0.190
10 – 32 UNF

59. Exercise 5-3
Write the thread note for a #10 fine thread. (See Appendix)
What is the minor diameter?
10 – 32 UNF

60. Exercise 5-3
Write the thread note for a #10 fine thread. (See Appendix E)
What is the minor diameter?
D – P =
0.190 – /32 =
0.156
10 – 32 UNF

61. Drawing Bolts D represents the major diameter.
Nuts are drawn in a similar fashion.

62. Bolt and Screw Clearances
Bolts and screws attach one material with a clearance hole to another material with a threaded hole.

63. Bolt and Screw Clearances
The size of the clearance hole depends on;
the major diameter of the fastener
and the type of fit
normal
close
loose

64. Bolt and Screw Clearances
Sometimes bolt or screw heads need to be flush with the surface. This can be achieved by using either a counterbore or countersink depending on the fasteners head shape.

65. Bolt and Screw Clearances
Counterbores: Counterbores are holes designed to recess bolt or screw heads below the surface of a part.
Typically,
CH = H + 1/16 (1.5 mm) and
C1 = D1 + 1/8 (3 mm)

66. Bolt and Screw Clearances
Countersink: Countersinks are angled holes that are designed to recess screws with angled heads.  
Typically,
C1 = D1 + 1/8 (3 mm)
Appendix E gives other counterbore, countersink and shaft clearance holes.